1. Technical Field
The present invention relates in general to earth-boring drill bits and, in particular, to a bit having a combination of rolling and fixed cutters and cutting elements and a method of drilling with same.
2. Description of the Related Art
The success of rotary drilling enabled the discovery of deep oil and gas reservoirs and production of enormous quantities of oil. The rotary rock bit was an important invention that made the success of rotary drilling possible. Only soft earthen formations could be penetrated commercially with the earlier drag bit and cable tool, but the two-cone rock bit, invented by Howard R. Hughes, U.S. Pat. No. 930,759, drilled the caprock at the Spindletop field, near Beaumont, Tex. with relative ease. That venerable invention, within the first decade of the last century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. The original Hughes bit drilled for hours, the modern bit drills for days. Modern bits sometimes drill for thousands of feet instead of merely a few feet. Many advances have contributed to the impressive improvements in rotary rock bits.
In drilling boreholes in earthen formations using rolling-cone or rolling-cutter bits, rock bits having one, two, or three rolling cutters rotatably mounted thereon are employed. The bit is secured to the lower end of a drillstring that is rotated from the surface or by a downhole motor or turbine. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drillstring is rotated, thereby engaging and disintegrating the formation material to be removed. The rolling cutters are provided with cutting elements or teeth that are forced to penetrate and gouge the bottom of the borehole by weight from the drillstring. The cuttings from the bottom and sides of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow, rotating drillstring, and are carried in suspension in the drilling fluid to the surface.
Rolling cutter bits dominated petroleum drilling for the greater part of the 20th century. With improvements in synthetic diamond technology that occurred in the 1970s and 1980s, the fixed-cutter, or “drag” bit, became popular again in the latter part of the 20th century. Modern fixed-cutter bits are often referred to as “diamond” or “PDC” (polycrystalline diamond compact) bits and are far removed from the original fixed-cutter bits of the 19th and early 20th centuries. Diamond or PDC bits carry cutting elements comprising polycrystalline diamond compact layers or “tables” formed on and bonded to a supporting substrate, conventionally of cemented tungsten carbide, the cutting elements being arranged in selected locations on blades or other structures on the bit body with the diamond tables facing generally in the direction of bit rotation. Diamond bits have an advantage over rolling-cutter bits in that they generally have no moving parts. The drilling mechanics and dynamics of diamond bits are different from those of rolling-cutter bits precisely because they have no moving parts. During drilling operation, diamond bits are used in a manner similar to that for rolling cutter bits, the diamond bits also being rotated against a formation being drilled under applied weight on bit to remove formation material. Engagement between the diamond cutting elements and the borehole bottom and sides shears or scrapes material from the formation, instead of using a crushing action as is employed by rolling-cutter bits. Rolling-cutter and diamond bits each have particular applications for which they are more suitable than the other; neither type of bit is likely to completely supplant the other in the foreseeable future.
Some earth-boring bits use a combination of one or more rolling cutters and one or more fixed blades. Some of these combination-type drill bits are referred to as hybrid bits. Previous designs of hybrid bits, such as is described in U.S. Pat. No. 4,343,371 to Baker, III, have provided for the rolling cutters to do most of the formation cutting, especially in the center of the hole or bit. Other types of combination bits are known as “core bits,” such as U.S. Pat. No. 4,006,788 to Garner. Core bits typically have truncated rolling cutters that do not extend to the center of the bit and are designed to remove a core sample of formation by drilling down, but around, a solid cylinder of the formation to be removed from the borehole generally intact.
Another type of hybrid bit is described in U.S. Pat. No. 5,695,019 to Shamburger, Jr., wherein the rolling cutters extend almost entirely to the center. Fixed cutter inserts 50 (FIGS. 2 and 3) are located in the dome area or “crotch” of the bit to complete the removal of the drilled formation. Still another type of hybrid bit is sometimes referred to as a “hole opener,” an example of which is described in U.S. Pat. No. 6,527,066. A hole opener has a fixed threaded protuberance that extends axially beyond the rolling cutters for the attachment of a pilot bit that can be a rolling cutter or fixed cutter bit. In these latter two cases the center is cut with fixed cutter elements but the fixed cutter elements do not form a continuous, uninterrupted cutting profile from the center to the perimeter of the bit.
A concern with all bits is stable running. Fixed- and rolling-cutter bits have different dynamic behavior during drilling operation and therefore different bit characteristics contribute to stable or unstable running. In a stable configuration, a bit drills generally about its geometric center, which corresponds with the axial center of the borehole, and lateral or other dynamic loadings of the bit and its cutting elements are avoided. Stabilizer pads can be provided to increase the area of contact between the bit body and the sidewall of the borehole to contribute to stable running. Such stabilizer pads tend to be effective in fixed-cutter bits, but can actually contribute to unstable running in rolling-cutter bits because the contact point between the pad and the sidewall of the borehole becomes an instant center of rotation of the bit, causing the bit to run off-center. Commonly assigned U.S. Pat. No. 4,953,641 to Pessier et al. and U.S. Pat. No. 5,996,731 to Pessier et al. disclose stabilizer pad arrangements for rolling-cutter bits that avoid the disadvantages of stabilizer pads. None of the foregoing “hybrid” bit disclosures address issues of stable running.
Although each of these bits is workable for certain limited applications, an improved hybrid earth-boring bit with enhanced stabilization to improve drilling performance would be desirable.